Production of fungitoxic extrolites was evaluated in culture filtrates of several isolates belonging to Penicillium canescens and P. janczewskii that showed some extent of inhibitory activity against the plant pathogenic fungus Rhizoctonia solani. In addition to griseofulvin and dechlorogriseofulvin that are already known in these species, curvulinic acid, previously unreported in Penicillium, was produced by all isolates assayed. Another extrolite recently characterized from a P. verrucosum strain by the name of Sch 642305 was detected in 5 isolates of P. canescens only. The purified compound completely inhibited mycelial growth of isolates of Rhizoctonia solani and other plant pathogenic fungi in vitro. The role of this extrolite as a possible biochemical determinant of antagonism toward plant pathogenic fungi, and implications concerning chemotaxonomy are discussed.